1. Field of the Invention
The present invention relates to a valve-regulated lead-acid battery and its production method, and more particularly to a valve-regulated lead-acid battery capable of contributing to improvement of penetration short-circuiting resistance and its production method.
2. Description of the Related Art
A valve-regulated lead-acid battery is designed to be tightly closed in a way to absorb oxygen gas generated in a positive electrode and to reduce to water at the time of charging in a negative electrode, and has characteristics that the battery does not require water supplementation and that the battery can be used in any posture and there are those known as the retainer type in which an electrolyte is retained in a retaining body just like a fine glass mat made of glass fibers with a mean fiber diameter of 1 μm or less and those known as the gel type in which an electrolyte is gelled. Although having a problem that retaining body just like a fine glass mat for retaining an electrolyte is costly, the retainer type ones tend to become popular in terms of exhibition of excellent effect for suppressing solution loss at the time of overcharging.
In recent years, the above-mentioned retainer type valve-regulated lead-acid battery has problems of cost down and high power output and with respect to the problem of cost down, grids of positive and negative electrode plates are produced in an expanding method to improve the productivity and with respect to the problem of high power output, the retaining body just like a fine glass mat is made thin to suppress voltage drop at the time of high power output. However, since the above-mentioned retaining body just like a fine glass mat has a low physical strength, in the case it is inserted between the positive and negative electrode plates using the grids produced in an expanding method, and it is affected by growth and deformation of the electrode plates, particularly of the positive electrode plate, the retaining body is broken and causes a problem of short-circuiting between the positive and negative electrode plates. To deal with that, it is tried to improve the physical strength of the above-mentioned retaining body by making the retaining body a mixed and scooped body of glass fibers and organic fibers.
On the other hand, with respect to the problem of high power output, the retaining body has to be durable to repeat of deep discharge, however in the case of using the mixed and scooped body of glass fibers and organic fibers, the valve-regulated lead-acid battery causes a problem that lead tends to be ionized and flow out of electrode plates when the concentration of the electrolyte in the retaining body is decreased by deep discharge, and accordingly eluted lead grows to be needle-like crystal at the time of next charging and penetrates the retaining body to result in penetration short-circuiting (dendrite short). It is supposedly attributed to that the mixed and scooped body of glass fibers and organic fibers is not being wetted evenly with the electrolyte because of an inferior solution retaining property as compared with that of a retaining body made glass fibers alone and a variety of means for improving the problem have been pursued (see Japanese Patent Application Laid-Open Nos. (JP-A) 2002-313305 and 7-29560).
JP-A 2002-313305 described above discloses a separator for lead-acid batteries containing glass fibers and organic fibers whose surfaces are coated with an inorganic oxide such as silica and aims to improve the strength without decreasing hydrophilicity and prevent fracture of the separator and penetration contact of electrodes even if the separator is made thin. Further, JP-A 7-29560 described above discloses a separator for sealed type lead-acid batteries obtained by mixing and scooping glass fibers, silica powder, and silica sol and aims to prevent the pressing power of the separator containing mainly glass fibers to the electrode plates from decreasing by reason of absorbing electrolyte, by mixing and scooping glass fibers with silica powder and silica sol in combination, as compared with the separator containing mainly glass fibers even if it absorbs the electrolyte.
It is reported that the separator disclosed in JP-A 2002-313305 described above can improve the short-circuit resistance and also improve hydrophilicity, however disclosed in Examples are only performance of separators obtained by laminating glass fiber mats and nonwoven fabrics of silica-coated polyolefin fibers (organic fibers), and in the case where such separators are used actually for valve-regulated lead-acid batteries and subjected to repeat test of deep discharge under conditions of practical use, whether the separators contribute to the improvement of the short-circuit resistance, or even if contributed to the improvement of the short-circuit resistance, whether the discharge capacity is sufficient or not is not disclosed at all. Further, with respect to the separators of JP-A 7-29560 described above, it is disclosed that glass fibers, silica powder, and silica sol are mixed and scooped to form gel which can contribute to retention of pressing power, and although the formation of the gel is a main point, it is not at all disclosed how the gel can be produced by adding glass fibers and silica powder to silica sol and mixing and scooping the mixture.
As described, even if a person skilled in the art can conceive of increase of short-circuit resistance owing to improvement of the hydrophilicity and contribution to improvement of the cycle life of the valve-regulated lead-acid batteries by retention of the pressing power owing to gel formation in the case of using the separators disclosed in JP-A 2002-313305 and 7-29560 described above, these patent documents do not suggest how to materialize the separators without hindering the productivity improvement during the valve-regulated lead-acid battery production process and it may be said that these patent documents do not contain such conception.